122 ME. C. E. TILLEY OX THE aRAlS^TE- GNEISSES [vol. Ixxvii^ 



There is a large development of green hornblende, although this is 

 not original, but developed paramorj)hically after the pyroxenes. 

 The accessories are magnetite and some biotite. The plagioclases 

 still show the original dusty inclusions. There has been no pro- 

 duction of garnet in this rock. The original rock was undoubtedly 

 of a quartz-doleritic tj^pe. 



(c) Chemical Changes induced by Metamorphism. 



The metamorphic changes which these dolerites have undergone 

 are essentially those of the highest grades of metamorphism. 

 A few remarks as to the probable nature and genesis of these 

 newly-formed minerals are necessary. It is clear that the original 

 rocks were essentially doleritic in character, the constituent minerals 

 of which as a group were plagioclase, augite, hypersthene, enstatite, 

 and olivine. Magnetite formed an accessory, present both in grains, 

 and as a dust to the primary pyroxenes. As a result of meta- 

 morphism, the original pyroxenes are recrystallized with a granu- 

 litic habit in which the dusty magnetite has been expelled, with 

 coalescence to newh^-formed grains. The felspars too have been 

 involved in the recrystallization, and the clear areas of this mineral 

 are indicative of a similar coalescence of inclusions. In some of 

 this recrystallized felspar the blastophitic texture has been retained; 

 but there has also been a reconstruction with destruction of this 

 texture, yielding a more typical granoblastic arrangement. 



With the appearance of garnet, chemical changes are involved, 

 and the same remark applies to the development of biotite aggre- 

 gating around grains of iron-ore. The production of garnet by 

 metamorphisin in rocks of this class is known to take place in 

 several ways. The possibilities are best expressed chemically, and 

 the reactions can be stated in the equations that follow : — 



(1) Interaction of felspar and pyroxene. 



CaMg(Si03)2 + CaAl,,Si20^ = Ca,MgAl^Si30i2+SiO,. 



(2) Interaction of felspar and olivine. 



Mg^SiO^ + CaAl^Si.O,, = CaMg2Al,Si30i2. 



^^^ "^MgAtsiof ' = CaMg-,Al,Si30,, + (a.-l)CaMg-SiA- 



(Aluminous ang-ite.) (G-arnet.) (Diopside.) 



Reactions of Types 1 and 2 have been used by Hezner ^ & 

 G-rubenmann ^ in their treatment of the eclogites, the typical garnet 

 of this rock beins; considered as a result of the interaction of 

 plagioclase with the metasilicate or orthosilicate molecule. In those 

 rocks the felspar-pyroxene interaction is attested by the associations 

 of these minerals, and receives further confirmation in the fact 

 that the inevitable bye-product of such a reaction (namely, silica) 

 is represented by quartz. 



1 Min. Petr. Mitth. vol. xsii (1903) p. 473. 



2 ' Die Kristallinien S chief er '1910. * 



